1. Field of the Invention
The present invention relates to a sheet conveying apparatus used in an image forming apparatus such as a copying machine, a printer, and a facsimile including a torque applying device that applies a predetermined torque on a rotary member and that adopts a method of fixing a non-fixed toner image by heating and pressuring, and an image forming apparatus such as the copying machine, the printer, and the facsimile equipped with the sheet conveying apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus using an electrophotographic system develops a latent image formed on a photosensitive drum serving as an image bearing member to obtain a visible image, and transfers the visible image (toner image) on a sheet using an electrostatic force. The toner image on the sheet is then fixed by heat to record and form the image on the sheet.
For such fixing device of the image forming apparatus, a heat roller fixing method of pressing a pressure roller having an elasticity against a fixing roller, which interiorly includes a heat source such as a heater to be maintained at a predetermined temperature, to form a fixing nip portion, and fixing the toner image on the sheet at the fixing nip portion is adopted.
Recently in the image forming apparatus (in particular, full color image forming apparatus) using this type of fixing device, a fixing device that can stretch the heating time and that can increase the fixing speed is known from the standpoint of enhancing the color emitting property and the image equality of the toner image. For example, as described in Japanese Patent Laid-Open No. 5-150679, a fixing device of a so-called belt nip method that presses an endless fixing belt bridged across a plurality of rollers against a heating roll is known.
Furthermore, higher speed is also necessary in the process speed in recent years to increase the speed of the output of the image forming apparatus. To this end, a wider nip width is required in a width direction orthogonal to a conveying direction of a sheet, and a belt fixing method that replaces the fixing roller or the pressure roller, or both rollers with an endless belt to ensure a wide nip width has been proposed and put into production (Japanese Patent Laid-Open No. 5-150679).
However, in a heat-fixing step of such fixing devices, heat and pressure are applied on the sheet, on which the toner image is transferred, and hence moisture evaporates from inside the sheet after the press nip portion and the press-nip. The change in the amount of moisture caused by the heat of the sheet and the stress caused by the pressure applied on the sheet that occurs at the time cause a phenomenon referred to as a curl, in which the sheet curls, or a phenomenon referred to as corrugation, in which the sheet becomes an undulated shape.
A sheet-like paper most typically used for the sheet is viewed in fiber level. The fibers are entangled to configure a paper, and moisture is contained inside the fiber or between the fibers. Furthermore, since the fiber and water are hydrogen bonded and are in an equilibrium state, smoothness is maintained.
However, if heat and pressure are applied on the sheet in the fixing step, shift occurs between the fibers by the pressure, and if heat is applied in such a state thus causing the moisture to evaporate, further hydrogen bonding occurs between the fibers thus leading to deformation. If such sheet is left untouched, the sheet absorbs moisture from the environment, whereby the hydrogen bonding between the fibers is again separated to return to the original state. However, the moisture does not go between the fibers at one part of the sheet, whereby the deformation of the sheet is maintained. The pattern of deformation includes the curl and the corrugation mentioned above, where the curl occurs due to the stretching/contracting difference of the front and the back of the sheet, and the corrugation occurs due to the stretching/contracting difference of the central part and the end of the sheet.
A first factor the corrugation occurs at the end of the sheet lies in a course in which the sheet is passed through the nip portion of the fixing device. For example, in the case of the fixing device including a wide nip as in the belt fixing method, a conveying velocity at the end side is set to be higher than at the central part in the width direction orthogonal to the conveying direction of the sheet in the nip to prevent wrinkles of the sheet in the course the sheet is passed through the nip portion. Thus, the end of the sheet stretches in the conveying direction than the vicinity of the middle after passing the nip portion when applying a rubbing action on the sheet, and the corrugation occurs at the end of the sheet.
A second factor the corrugation occurs at the end of the sheet lies in after the sheet is passed through the nip portion of the fixing device. In a state mounted as a sheet bundle, the respective sheets are brought into contact with the atmosphere at the end, and thus entering and exiting of the moisture rapidly occur. If the heat is applied on the sheet in the fixing step thus evaporating the moisture inside the sheet, and then the moisture is rapidly absorbed from the end of the sheet, the end of the sheet stretches in the conveying direction than the vicinity of the middle, as expected, and the corrugation occurs at the end of the sheet.
In particular, in the belt fixing method in which the fixing roller or the pressure roller, or both rollers are replaced with the endless belt to ensure the wide nip width, the distance and the time the sheet resides between the nips are increased compared to the heat roller method, whereby the problem of corrugation at the end of the sheet becomes significant.
According to the review from the inventors of the present application, it was found that the corrugation of the sheet can be corrected by pulling (applying tension to) the central part in the width direction of the sheet. In a more specific configuration, it was found that the corrugation of the sheet can be corrected by applying a load torque on the rotation of a pair of rollers on the upstream side when conveying the sheet by the pair of rollers on the upstream side and a pair of rollers on a downstream side.
Generally, when applying the load torque on the rotation of the roller (rotary member), a powder seal member, a bearing, and the like, which are configuring components, of a powder brake, for example, become high temperature during the high-rotation continuous use. The torque then may rapidly increase, the durability may degrade, or breakage may occur in the powder brake, and the like. Thus, a usage temperature limit (lower than or equal to 80° C.) is given to the powder brake, and the powder brake needs to be used at the torque and the rotation number that do not exceed the usage temperature limit.